Many popular digital sound formats utilize lossy data compression techniques that discard some of the data to reduce storage or data rate requirements. The application of lossy data compression not only reduces the fidelity of source content (e.g., audio content), but it can also introduce noticeable distortion in the form of compression artifacts. In the context of audio coding systems, these sound artifacts are called coding noise or quantization noise.
Digital audio systems employ codecs (coder-decoder components) to compress and decompress audio data according to a defined audio file format or streaming media audio format. Codecs implement algorithms that attempt to represent the audio signal with a minimum number of bits while retaining as high a fidelity as possible. The lossy compression techniques typically used in audio codecs work on a psychoacoustic model of human hearing perception. The audio formats usually involve the use of a time/frequency domain transform (e.g., a modified discrete cosine transform—MDCT), and use masking effects, such as frequency masking or temporal masking so that certain sounds, including any apparent quantization noise is hidden or masked by actual content.
Most audio coding systems are frame based. Within a frame, audio codecs normally shape the coding noise in the frequency domain so that it becomes least audible. Several present digital audio formats utilize frames of such long durations that a frame may contain sounds of several different levels or intensities. Since the coding noise is usually stationary in level over the evolution of a frame, coding noise may be most audible during low intensity parts of the frame. Such an effect may be manifested as pre-echo distortion in which silence (or low-level signal) preceding a high intensity segment is swamped by noise in the decoded audio signal. Such an effect may be most noticeable in transient sounds or impulses from percussion instruments, such as castanets or other sharp percussive sound sources. Such distortion is typically caused by the quantization noise introduced in the frequency domain being spread over the entire transform window of the codec in the time domain.
Present measures to avoid or minimize pre-echo artifacts include the use of filters. Such filters, however, introduce phase distortion and temporal smearing. Another possible solution includes the use of smaller transform windows, however this approach can significantly reduce frequency resolution.
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